The British government promised to protect America's interests during the Chilcot inquiry into the Iraq war, according to a secret cable sent from the US embassy in London.

Jon Day, the Ministry of Defence's director general for security policy, told US under-secretary of state Ellen Tauscher that the UK had "put measures in place to protect your interests during the UK inquiry into the causes of the Iraq war".

The admission came in the cable sent on 22 September 2009, which recorded a series of high-level meetings between Tauscher and UK defence officials and diplomats, which involved the then foreign secretary, David Miliband.

Day was a senior adviser to the Labour government, and told the American delegation that "Iraq seems no longer to be a major issue in the US", but said it would become a big issue – a "feeding frenzy" – in the UK "when the inquiry takes off".

The revelation of the move to defend Washington threatens to undermine the inquiry, which was launched by Gordon Brown 'to identify lessons that can be learned from the Iraq conflict'. It is due to deliver its findings around the turn of the year.

The diplomats do not record which measures the British government took to protect US interests. No American officials were called to give evidence in public, and evidence from US officials was heard in private during visits by inquiry members to the US. The inquiry was also refused permission to publish letters between George Bush and Tony Blair written in 2002 in the run-up to the war, even though they were referred to in evidence. There were fears that the release of the details could harm both UK-US relations, and those with other countries. In January, a Blair ally told the Guardian: "They are full of scurrilous remarks about other people, including [Jacques] Chirac [the former French president]."

Tonight, Andrew Burgin, a spokesman for the Stop the War Coalition, was reported as saying: "This shows is the beginning of the cover-up".

"This really brings the whole inquiry into disrepute," he said. "Those involved in this cover-up must be held to account. The implications are so serious that there may need to be a new inquiry."